1. Field of the Invention
This invention relates generally to an antenna system for a vehicle radio and, more particularly, to a backlite antenna system for a vehicle radio in which antenna elements are embedded in a rear window of the vehicle and are separate from defogger elements embedded in the rear window, and where the antenna system provides broadband FM reception.
2. Discussion of the Related Art
Most vehicles include a vehicle radio that requires some type of antenna system to receive amplitude modulation (AM) and frequency modulation (FM) broadcasts from various radio stations. Most present day vehicle antenna systems include a mast antenna that extends from a vehicle fender, vehicle roof, or some applicable location on the vehicle. Although mast antennas provide acceptable AM and FM reception, it has been recognized by vehicle manufacturers for many years that the performance of a mast antenna cannot be further enhanced, i.e., provide increased AM and FM reception capabilities over the current state of the art reception. Therefore, improvements attained in all other areas of in-vehicle entertainment systems will not include reception capabilities of the mast antenna. Consequently, car manufacturers have sought other types of antenna designs to keep pace with demands in increased vehicle stereo and radio capabilities.
Improvements in vehicle antenna systems have included development of backlite antenna systems in which antenna elements are embedded in a rear window of the vehicle in various manners. As is understood, such a backlite antenna system can have improved reception performance for both AM and FM reception over mast antenna systems. Backlite antenna systems have also provided a number of other advantages over mast antenna systems, including, no wind noise, reduced drag on the vehicle, elimination of corrosion of the antenna, no performance change with time, no risk of vandalism, and reduced cost of installation.
Typically, known backlite antenna systems utilize defogger elements already encapsulated in the back window of the vehicle as antenna elements to receive the AM and FM broadcasts. Examples of such backlite antenna systems can be found in U.S. Pat. No. 5,293,173 issued to Kropielnicki, et al. Mar. 8, 1994, and U.S. Pat. No. 5,099,250 issued to Paulus, et al. Mar. 24, 1992. For the known combination defogger/antenna element systems embedded in rear windows of vehicles, it has been necessary to incorporate two bifilar or toroidal chokes between the elements and the vehicle DC power supply so as to separate the antenna signals from the high current signals that heat the elements. These chokes provide low impedance paths for the propagation of the relatively large current flow necessary to power the elements, and a high impedance path against the propagation of the radio signals. A first choke of a relatively small inductance is generally used for the FM range, and a second choke having a much larger inductance, generally greater than 1 mH, is generally used for the AM range. For lower frequencies, the impedance of a typical heater element relative to the metal of the vehicle body approaches that of its capacitance. The use of the choke is important to eliminate the DC magnetism present from this capacitance.
Use of these types of chokes to separate the antenna signals from the high current signals that heat the defogger/antenna elements includes a number of disadvantages for these types of known backlite antenna systems. Particularly, the chokes are heavy, expensive and cumbersome to implement. Therefore, use of such chokes has been less than desirable.
Other disadvantages of utilizing already existing defogger elements as antenna elements of an antenna system also exist. For one particular disadvantage, the length of the defogger elements across the width of the window sometimes causes end bus bar elements to contact conductive glue that secures the window to the vehicle body, or cause the bus bar elements to be close to the sheet metal of the vehicle body. This creates a capacitance to ground connection resulting in a degradation of AM performance of the antenna elements. Therefore, the defogger elements must be reduced in length (positioned inboard) for proper AM performance for certain vehicle body styles. This reduction in length makes the end bus bars visible on the window, reducing its aesthetic appearance. Therefore, some times additional costs are incurred to change trim members around the window to cover these end bus bars.
Another drawback of utilizing already existing defogger elements as antenna elements has to do with high currents that are desirable, on the order of 30 amps, for quickly defogging the window of the vehicle. These high currents are applied to the matching network of the antenna elements. Because high currents will adversely effect at least some of the components in the matching network, this current is limited to a maximum current. This maximum current may be below what is desired for a particular defogging application.
The bifilar chokes used in the prior art backlite antenna systems are generally incorporated in an antenna impedance matching network. The impedance matching network is necessary in these types of antenna systems to match the output of the antenna elements to the input of an amplifier associated with the vehicle radio so as to reduce the attenuation of power transfer from the antenna elements to the radio. Known impedance matching networks typically have not been universal in that the network components or network design must be changed from vehicle to vehicle to realize the greatest efficiency in impedance matching. This is because the capacitance created between the elements and the vehicle body varies from vehicle to vehicle. Further, prior art antenna grid patterns are directional at FM frequencies, and have low gain at AM frequencies.
FM transmissions in the United States are within a well regulated frequency range of 88 MHz to 108 MHz. Therefore, antennas associated with vehicles operating in the United States are tuned to this particular bandwidth. However, other countries may operate within different regulated bandwidths for their FM transmissions. For example, Japan regulates FM transmissions in the frequency bandwidth of 76 MHz to 90 MHz. Therefore, antennas associated with vehicles operating in Japan are tuned to this particular bandwidth. It would therefore be desirable that vehicles that are sold and/or operated in both the United States and Japan would include an antenna that was tuned to FM broadcasts in the bandwidth of 76 MHz to 108 MHz so as to eliminate the need to provide different antenna systems for the same type of vehicles operated in both the U.S. and Japan.
What is needed is a backlite antenna system for an AM/FM vehicle radio that does not include chokes, includes an antenna responsive to a wide bandwidth for operation in multiple countries, and includes a matching network that can be incorporated into a wide variety of vehicles. It is therefore an object of the present invention to provide such an antenna system.